Determining a location and identity of a buried asset can be a challenging task. In some cases, determination of the location is performed by systematically digging holes in ground until the asset is found. In some cases, ground penetrating radar (GPR) is used, assets being identified by a signal reflected by the asset. (Reference to GPR includes radiation having a frequency in the range of from around 200 MHz to around 1 GHz. Other frequencies are also useful).
This technique for locating buries assets suffers the disadvantage that radiation can be reflected by a number of features of a volume of ground, including variations in moisture content, solids composition, the presence of wildlife, and voids formed for example by tunnelling wildlife. Thus it can be difficult to reliably identify a location of a buried asset.
Acoustic techniques have also been used to identify buried assets and found to suffer similar disadvantages to GPR techniques.
It is also known to attach a radio frequency identification tag (RFID tag) to a non-buried asset such as items for sale in a store in order to enable identification of the asset using a suitable RFID tag reader. Typically, an RFID tag is placed in an inductive RF magnetic field to induce a flow of electrical current in the tag. The flow of current is used to generate an RF transmission by the tag to the reader.
Inductive magnetic fields are relatively short range, however, and the use of such RFID tag systems to locate and identify assets more than a few meters from the reader is generally not practical.
In the case of buried assets the problem is exacerbated since inductive RF magnetic fields are more strongly attenuated by soils than by air.
More recently, far-field RFID technology has been developed. However, RFID readers are incapable of generating sufficiently large excitation signals to detect RFID tags at required distances below a surface.
U.S. Pat. No. 3,769,623 discloses a dichroic plate for passing radiation within a particular frequency band and reflecting radiation outside of that frequency band. The dichroic plate has slots provided therein dimensioned to pass radiation of a selected frequency.
U.S. Pat. No. 5,837,926 discloses mines having tuned passive electromagnetic reflectors to enhance radar detection.